AND8031/D

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2

Since the power level is low, an RC EMC filter is utilized.

The RC filter is not as efficient as an LC, but it uses less

board area and will cost less. 1N4005 600 volt fast diodes

provide the full wave rectification of the AC input. The bulk

capacitor is the filter. The selection of the capacitor is

determined from three factors: input voltage, ripple current

and maximum output ripple. The equations for each are as

follows.

Vdc

+ Vac · 2

(eq. 1)

Irms

+

Ipk2 · D

3

(eq. 2)

(Discontinuous

mode only)

Cin

+

k·Pin (AV)

f · (Vripple (p–p))2

(eq. 3)

Pin

+ Pout

h

(eq. 4)

Where k is 1 for AC inputs the peak to peak ripple is 6.0

volts and

η is estimated to be 70% with the low power level

of the board and the RC input filter. Solving for the equations

above and using 50% for the maximum duty cycle, Vdc

maximum is 375 volts, Vdc minimum is 120 volts, Ipeak and

Irms are solved for in the transformer calculations, Pin is 3.2

The transformer converts the rectified line voltage to the

output voltage and is controlled by the MC33363B. To

design the transformer, the following data is necessary. Vdc

minimum is 100 volts (allowing for bulk voltage ripple and

power switch voltage drop), the frequency is 100 kHz, the

maximum duty cycle of the power switch and the reset time

for the secondary are both 45% to maintain discontinuous

conduction, and the secondary diode forward voltage drop

is 0.45 volts (estimated).

Psec

+ (Vout ) Vdiode) · Iout

(eq. 5)

Pin

+

1

2

Lpri · T

(eq. 6)

, or

Lpri

+

1

2

Pin · T

(eq. 7)

Ipeak

+

2·P· T

L

(eq. 8)

Lpri

Lsec

+

Np

Ns

2

(eq. 9)

For the primary side of the transformer, Lpri is 3.16 mH,

Ipeak is 142 milliamperes, ton is 4.5 usec and Irms is 55

milliamperes–rms. For the secondary, Psec is 2.25 watts, L

is 2.28 uH, Ipeak is 4.44 amperes and Irms is 1.72

amperes–rms. The turns ratio is 37.2. The auxiliary winding

is set to 12 volts and 10 milliamperes. Using the secondary

turns ratio, the auxiliary turns ratio is calculated as 6.6 in

reference to the primary.

With the peak primary current and the frequency

determined, the components surrounding the MC33363B

can be selected. Referring to Figures 2 and 3, RT will be set

to 30 k Ohms and CT will be set to 390 pF. The Over Voltage

Protection, pin 11, will not be used. Its main function is for

loss of optocoupler protection. Pin 8, the reference voltage,

requires a 1.0 uF ceramic capacitor for stability. The voltage

compensation and voltage feedback, pins 9 and 10, will be

covered in the feedback section of this document. The Vcc

pin requires a 10 uF capacitor for stability.

1.0 M

7.0

500 k

200 k

100 k

50 k

20 k

10 k

10

15

20

30

50

70

0.1

0.8

0.6

0.4

0.2

0.15

7.0

10

15

20

30

40

70

50

0.3

1.0

Figure 2. Oscillator Frequency

versus Timing Resistor

Figure 3. Power Switch Peak Drain Current

versus Timing Resistor

Inductor supply voltage and

inductance value are

is achieved at 5.0 ms.